Group III nitride crystal, which is a semiconductor, is used in the field of, for example, heterojunction high-speed electronic and photoelectronic devices (a semiconductor laser, a light emitting diode (LED), a sensor, etc.). Gallium nitride (GaN) crystals have attracted particular attention. Conventionally, a single crystal of gallium nitride has been produced by causing gallium and nitrogen gas to react directly with each other (see Non-Patent Document 1). However, this reaction requires ultra-high temperature and pressure conditions, i.e., 1300 to 1600° C. and 8000 to 17000 atm (about 800 to 1700 MPa). To solve this problem, a technique of growing a gallium nitride single crystal in a sodium (Na) flux (melt) (hereinafter also referred to as a “Na flux method”) has been developed (see, for example, Patent Documents 1 to 3 and Non-Patent Documents 2 and 3). By this method, the heating temperature can be reduced significantly to 600 to 800° C., and the pressure can be reduced to as low as about 50 atm (about 5 MPa). In the Na flux method, a seed crystal is placed in the Na flux beforehand so that a crystal is grown from the seed crystal as a nucleus. A bulk crystal can be obtained by this method. When a single GaN crystal is grown, the seed crystal is, for example, a multilayer substrate in which a thin GaN crystal film layer is formed on a sapphire substrate using MOCVD or HVPE. Such a technique is also referred to as “liquid phase epitaxy.”
The application of light emitting diodes (LED), which are electronic devices made of single GaN crystals, to lighting has been expected and, therefore, has been studied and developed actively. This is because an LED made of a single GaN crystal can save significant power as compared to conventional lighting devices, such as fluorescent lamps and the like. However, there is a limitation on improvement of the luminance of a conventional GaN single-crystal LED and, therefore, a high luminance, which theoretically is predicted, has not been achieved. This is because there are the following three problems:
(1) the non-uniform distribution of charge in the LED;
(2) a change in the wavelength of emitted light due to an increase in luminance; and
(3) the presence of a high-resistance portion of the LED.
The three problems (1) to (3) can be solved theoretically by fabricating an LED on a non-polar surface of a GaN crystal substrate. Non-polar surface refers to a surface on which charge is uniformly distributed. However, in the conventional art, it is not possible to fabricate an LED on the non-polar surface of a GaN crystal substrate. FIG. 1 shows a basic structure of an LED. As shown in FIG. 1, the LED includes a low-temperature buffer layer 2, a GaN crystal (Si-doped) layer 3, an n-AlGaN (Si-doped) layer 5, an InGaN layer 6, a p-AlGaN (Mg-doped) layer 7, a p-GaN (Mg-doped) layer 8, a transparent electrode layer 9, and a p-layer electrode 10, which are laminated in this stated order on a substrate 1 made of sapphire or silicon carbide. In addition, an n-layer electrode 4 is provided on a portion of the GaN crystal (Si-doped) layer 3. In the fabrication of a conventional LED, as show in FIG. 2, the GaN crystal layer 3 is grown on the substrate 1 made of sapphire or the like by liquid phase epitaxy with the low-temperature buffer layer 2 being interposed therebetween. The GaN crystal is grown only on its c-plane. Although a side surface 32 of the GaN crystal layer 3 is an M-plane, an upper surface 31 of the GaN crystal layer 3 is a c-plane. If a non-polar surface, such as an M-plane or the like, is forcibly formed, the resultant crystal is of poor quality, so that the luminance cannot be improved.
Moreover, in liquid phase epitaxy using Na flux (melt), nuclei other than the seed crystal occur in the flux, which leads to a deterioration in the quality and yield of a GaN crystal. FIG. 8 shows a graph indicating an exemplary relationship between the pressure of nitrogen gas and the thickness (film thickness) of a GaN crystal formed on a substrate. As shown in FIG. 8, as the nitrogen gas pressure is increased, the GaN crystal thickness also increases. When the nitrogen gas pressure exceeds some point (in FIG. 8, about 34 atm), the GaN crystal thickness conversely decreases. This is because, as shown in FIG. 8, nuclei other than the seed crystal occur in the Na flux.
The non-polar surface growth problem and the nucleation problem should be solved not only for the production of a GaN crystal, but also for the production of other group III nitride crystals.    Patent Document 1: JP 2000-327495 A    Patent Document 2: JP 2001-102316 A    Patent Document 3: JP 2003-292400 A    Non-Patent Document 1: J. Phys. Chem. Solids, 1995, 56, 639    Non-Patent Document 2: Journal of the Japanese Association for Crystal Growth, 30, 2, pp 38-45 (2003)    Non-Patent Document 3: J. J. Appl. Phys. 42, pp L879-L881 (2003)